pinctrl: sh-pfc: r8a7795: Support none GPIO pins with configurable drive-strength
There are pins on the r8a7795 which are not part of a GPIO bank nor
can be muxed between different functions. They do however allow for the
drive-strength to be configured. Add those pins to the list of pins and
to the drive-strength configuration registers.
The pins can now be referred to in DT by function names and their
drive-strength modified.